J keeps a log of interesting events in the file ~/.j/log (or C:\.j\log). By default, only errors are logged.

When debugging is enabled, debugging information is also logged, and all output sent to the log file is also sent to System.err, so it can be seen in the console window from which j was started.

You can enable debugging by adding the following line to your preferences file:

    debug = true

The maximum size of the log file is controlled by the property Log.maxFileSize; the default is 100 KB. When the log file exceeds its maximum size, it is rolled over to the file ~/.j/log.0, which in turn is rolled over to ~/.j/log.1, etc. The maximum backup index is controlled by the property Log.maxBackupIndex; the default is zero, which means one backup is made.

To disable logging entirely, add this line to your preferences file:

    Log.enabled = false

If logging is disabled, nothing will be written to the log file, but if debugging is enabled, log output will still be sent to System.err.